This invention relates to electrical insulation for devices having electrical windings, and more particularly to electrical insulation which has specifically improved temperature stability and performance characteristics.
Electrical coils, transformers, and magnet devices used at or near room temperature are typically insulated using varnish insulation or other easy to apply polymer coatings. Higher temperature devices use stronger and more stable plastics in order to accommodate an upper limit operating temperature of approximately 200xc2x0 C. (390xc2x0 F.). Superconducting coils and magnets must withstand temperature fluctuations from above room temperature down to almost xe2x88x92270xc2x0 C. (xe2x88x92454xc2x0 F.) and therefore typically use advanced glass fiber reinforced epoxy resins. However, none of these insulation systems is capable of surviving manufacturing or use temperatures above approximately 250xc2x0 C. (480xc2x0 F.).
Improved energy efficiency can often be obtained by raising the operating temperature of electrical coils and transformers. Production and operating costs can also be lowered with the elimination of cooling systems. The possible applications for these devices could be greatly multiplied if the need for protective coverings and thermal shielding from the hot sun or other heat producing machines could be eliminated.
In many instances, the unavailability of a suitable high temperature insulation material creates high production costs and inefficiencies. For example, some niobium tin superconducting magnets are wound and then heat treated at approximately 600 to 800xc2x0 C. In the state of the art production processes, the coil then must be carefully unwound slightly to allow for the wrapping of glass reinforced epoxy insulation. Finally, the coil is rewound into the final shape. Clearly, this complicated process greatly increases the final costs of the product. A high temperature wrappable insulation, which could be applied before the niobium tin superconducting magnets were wound and heat treated, would save tremendous amounts of labor and time, increasing productivity, reducing loss, and resulting in greatly decreased final product costs.
Some prior art approaches involve using alternative insulation materials to increase the temperature limits of electrical coils, transformers and magnets. For example, ceramic coatings and layers have been applied in specialty applications to raise the maximum temperature limits to very high levels. Most of the examples of prior art discussed below involve the special case of superconducting coils and magnets. However, the principles and disadvantages described are equally applicable to normal metal wire coils, transformers and magnets.
U.S. Pat. No. 5,336,851 to Sawada et al. discloses methods for insulating an electrical conductor wire having a high operating temperature by placing up to three layers of ceramic particles around the conductor. Applying multiple ceramic layers can be complicated and add cost to the conductor. Also, the particles produced are very weakly bonded together because the processing temperature cannot be raised high enough to fuse them without melting the electrical conductor.
U.S. Pat. No. 5,139,820 to Sawada et al. describes a method of manufacturing ceramic insulated wire by extruding an inorganic gel around the conductor. However, gels typically shrink significantly upon densification (20 to 50 volume percent or more) to their final state. This shrinkage can cause cracks in the insulation or change the desired dimensions of the coil.
U.S. Pat. No. 5,212,013 to Gupta et al. discloses methods for insulating superconducting wire with an inorganic glass ceramic composite system wire insulation. The problem with this approach is that the composition of this system would need to be modified for each heat treatment temperature desired for the superconductor. The glasses melt in a narrow temperature range and would only have the desired viscosity in that same narrow range. Too high a viscosity and the insulation would not fuse into a continuous layer, ruining the electrical insulation properties. Too low a viscosity and the composite would flow, allowing the conductors to move and possibly touch, again ruining the electrical insulation properties. This system would be complicated to apply in a manufacturing environment, to the point of impracticality.
William N. Lawless, a co-inventor of U.S. Pat. No. 5,212,013, states in his paper Dielectric Insulations Incorporating Thermal Stabilization for A-15 and Ceramic Superconductors, presented at IECEC-98-039 in Aug. 1998 that glass is the only other viable co-firable material for insulating superconductors.
U.S. Pat. No. 4,429,007 to Bich et al. discloses electrical wire insulation for an electromagnetic coil, wherein the wire is coated with a ceramic powdered slurry. This method has the same narrow temperature range as described in U.S. Pat. No. 5,212,013, and the composition must also be changed if the heat treatment of the superconductor is at a temperature other than 750xc2x0 to 790xc2x0 C. In addition, the 14-step heating process as outlined is long and involved, adding unnecessary expense to the final product.
Enhanced electrical insulation is needed to take advantage of many new developments in the field of superconducting magnets. The higher temperature processing required for xe2x80x9cA-15xe2x80x9d compounds (e.g. niobium-tin and niobium-aluminum) and oxide (high temperature) superconductors makes compatibility with the insulation even more difficult. New insulation demonstrating increased strength and modulus would substantially improve magnet performance. In addition, insulation capable of surviving wind and react processing would significantly lower cost.
Many magnet designs require an insulate-before-winding approach in order to achieve top performance. Due to bend strain limitations of A-15 and HTS conductors, a wind-before-react technique is required for complex shapes with tight bends (saddle coils or dipoles for MHD, motors, and generators). Niobium tin and oxide superconductors are inherently brittle after heat treatment. All handling performed in this brittle state must limit the strain applied to a fraction of one percent. For example, the current value being used in industry for ITER (International Thermonuclear Experimental Reactor) type magnets is 0.1% strain. In order to obtain the desired conductor placement and turning radii, the conductor must be shaped and wound into the coil prior to heat treatment. However, current high performance organic insulation cannot survive these conditions. Therefore, magnet manufacturers have had to very carefully xe2x80x9cunwrapxe2x80x9d the magnet after heat treating in order to insulate it. As described supra, this step adds extra cost as well as limits the ultimate design.
To summarize the current state of the art, as described above, we have provided a listing of the problems presently encountered with current organic and inorganic insulation approaches:
Problems with current organic insulation
(a) Coils manufactured with organic insulation systems must have all the high temperature processing completed before the insulation can be applied. This limits the fabrication of some devices and increases the cost of others.
(b) The maximum temperature during operation is limited by the temperature stability of the organic insulation. Design changes or additional costs associated with cooling or thermal barriers are
(b) The maximum temperature during operation is limited by the temperature stability of the organic insulation. Design changes or additional costs associated with cooling or thermal barriers are required to operate above these limits, which are quite low, typically no more than about 200xc2x0 C.
(c) Devices made with organic insulation are more susceptible to damage from ionizing and non-ionizing radiation. Useful lifetimes are diminished if additional radiation shielding is not used.
Problems with current inorganic insulation
(a) Prior art inorganic insulation systems suffer from complex processing methods, such as plasma spraying, that make them difficult to apply to thin wires or conduits. Applying multiple layers on a wire are more expensive than single layers.
(b) Some ceramic insulation systems use particulate ceramic powders. However, in order to achieve high strength and electrical isolation, very high temperatures are required during processing that will melt the metal in the conductor. Superconductor materials are heat treated at too low of a temperature (600xc2x0 C. to 1000xc2x0 C.) to allow these powders to sinter together and achieve the desired properties.
(c) Some glass insulation systems use particulate glass powders. These often have a very limited processing temperature range where the insulation is fluid enough to fuse together but not too fluid to flow out from between the wires. Both too little and too much flow will lower the performance of the device.
(d) Glass insulation systems also have a narrow range of
(e) The narrow range of composition mentioned above limits the addition of thermal control additives to improve properties such as thermal conductivity or specific heat. Several ceramic powders have been identified that possess enhanced thermal performance at specific temperatures (such as 4 K to 8 K) for superconducting magnets (see, for example, U.S. Pat. No. 5,212,013 to Gupta). The use of any of these components would upset the processing temperature range of glass insulation and require reformulation.
(f) Ceramic insulation systems that extrude a gel or mixture of powders are hard to reinforce with continuous fibers. The mechanical strength of the coil or magnet is lower than if a fabric could be used. The shrinkage associated with the densification of sol-gel insulation systems will generate cracks around the metal wire and any fiber reinforcement. Cracks will lower the electrical and mechanical strength of this type of insulation.; and
(g) Solid ceramic insulation is brittle after application on the wire or conductor. Applying the insulation in its final form prior to winding the coil will limit the radius of curvature that can be achieved without cracking the insulation. Tight, small coils cannot be made with pre-applied, dense ceramic insulation.
The preceramic polymer insulation invention described herein allows conducting coils and magnets to be fabricated using existing processing equipment, and maximizes the mechanical and thermal performance at both elevated and cryogenic temperatures. It also permits co-processing of the wire and the insulation to increase production efficiencies and reduce overall costs, while still remarkably enhancing performance.
More particularly, there is described herein a high temperature electrical insulation suitable for electrical windings for any number of applications. The insulation comprises a cured preceramic polymer resin, which is preferably a polysiloxane resin made by Allied Signal and marketed under the trademark BLACKGLAS.
In another aspect of the invention, there is described a method for insulating electrical windings which are intended for use in high temperature environments, such as superconductors and the like. This method advantageously comprises the steps of, first, applying a preceramic polymer layer to a conductor core, to function as an insulation layer, and second, converting the initial preceramic resin into ceramic insulation, by curing the preceramic polymer layer. Of course, the conductor core preferably comprises a metallic wire, which may be wound into a coil. In the preferred method, the applying step comprises a step of wrapping the conductor core with a sleeve or tape of glass or ceramic fabric which has been impregnated by a preceramic polymer resin. In some embodiments, the preceramic polymer resin may be comprised of a polymer selected from the group consisting of polysilazanes, polycarbosilanes, polysiloxanes, polysilsesquioxanes, polyaluminosiloxanes, polyaluminosilazanes and polymetallosiloxanes. For certain applications, wherein a polysilazane polymer is chosen, the preceramic polymer may be selected from the group consisting of hydridopolysilazanes, silacyclobutasilazanes, boron-modified hydropolysilazanes, and vinyl-modified hydridopolysilazanes. Presently, it is preferred that the preceramic polymer is a polysiloxane resin sold under the trademark BLACKGLAS, and available from Allied Signal.
However, for other applications, the preceramic polymer may be selected from the group consisting of a spirosiloxane oligomer, a spirosiloxane polymer, and a polyvinylsilane, impregnated with said preceramic polymer resin by pouring or spraying the resin onto the sleeve or tape.
The present application is particularly advantageous because it comprises a ceramic composite insulator, suitable even for the harsh superconducting magnet environment, which combines the ease of processing of conventional organic insulation, but is also capable of withstanding the same heat treatment as the conductor itself. Even more beneficial, the present ceramic insulation may be applied in the same way as conventional organic insulation, using pre-preg tapes made from preceramic polymers. The attraction of a ceramic pre-preg that could be fired at the same time that the superconducting wire is being reacted is two-fold. First, it saves much time and expense by reducing processing steps and costs. By wrapping the ceramic onto the conduit, the same equipment used today for organic pre-preg insulation can be re-used. Second, more design flexibility is afforded, thereby allowing higher performance magnet coils to be fabricated.
The invention, together with additional features and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying illustrative drawing.